(i) Field of the Invention
This invention relates to systems for launching, towing and recovering an oceanographic towed body. The invention relates generally to a towing system wherein an oceanographic body is towed behind a vessel. The system normally includes a type of shock-absorbing system for the tow line which is not harmful to such tow line; means for accommodating abnormal or erratic tow line behaviour without wrecking the system structure or harming the tow line; means for keeping the fleet angle near zero during winding the cable onto and off the winch during paying out and winding in of the towing cable; means for training the fairings surrounding the tow cable as the cable is wound onto the winch drum to prevent damage or destruction of such fairings; and apparatus for launching and recovering the towed body without resorting to mechanical interlocking between the body and a member engageable with the body during launching and recovering. By "fleet angle" is meant the angle between the cable run immediately prior to reaching the winch drum and a theoretical line drawn at right angles to the winch drum axle when viewed normal to a plane encompassing both cable run and winch drum axle.
The underwater towed body with which the present invention is used is an underwater SONAR (abbreviated from "Sound Navigation and Ranging") which is finding ever-increasing use in the fields of navigation, mapping, depth finding, fish finding, for detection of wrecks and in a military use, in the detection of enemy vessels. The system with which the present invention is used is a variable depth system, wherein an underwater sound transducer or array is mounted on a body towed from a vessel.
In a variable depth sonar, particularly as used in military applications, an array (usually cylindrical) of underwater sound transducers is housed within a streamlined body which is towed from the surface ship via a faired cable. This cable has an internal core of conductors for transmitting signals to and from the ship from and to the array, and outer layers of armour to withstand towing tensions. Mounted on the ship is means for mechanically launching and retrieving the body, and for shortening and lengthening the amount of cable out.
With respect to the problems of launching and recovering, in the towing system as described, it is necessary to make provision for launching the body prior to towing, and retrieving the body at the conclusion of towing, both in a manner to prevent damage to the towing cable and the object being towed. The launch operation comprises lifting the sonar body from a stowed position on or above the deck of the ship, deploying it over the stern until it is largely immersed in the water, then releasing it to stream aft below the surface. The recovery operation comprises the reeling in of the towed sonar body until it is captured in the launch and retrieval mechanism, then deploying it aboard by operating the hoist.
(ii) Description of the Prior Art
Various means have been developed to launch and retrieve a towed body. One means in common use is described in Canadian Pat. No. 879,530 issued Aug. 31, 1971 to R. L. I. Fjarlie et al. This means utilizes a saddle to hold the body captured in compressive contact against the underside of the saddle during launch and recovery, a boom pivoting at its inboard end from a hoist frame mounted on the deck of the ship adapted for supporting the saddle and a towing sheave at its outboard end, a pair of pantograph arms to hold the saddle level during launch and recovery, and hydraulic cylinders mounted on the hoist frame and attached to the boom. Hydraulic pressure introduced into either the head or rod ends of the cylinder is used to drive the boom out for launch, or to drive it in for body recovery. Boom motion is in the form of a long overhead arc in a vertical plane parallel to the fore and aft centreline of the ship.
Somewhat different is the method utilized in Canadian Pat. No. 1,005,702 issued Feb. 22, 1977 to A. Kemeny, wherein the boom is supported at its inboard end on a rotary actuator which replaces the cylinders in Canadian Pat. No. 879,530. In addition, a system of cables and sheaves replaces the pantograph arms to hold the saddle level during launch and recovery. Pressure within the rotary actuator is used to drive the boom out for launch and in for recovery. Like Canadian Pat. No. 879,530, boom motion is in the form of a long overhead arc in a vertical plane more or less parallel with the centreline of the ship.
In both of the foregoing patents, the body is held level in smooth seas during launch and recovery.
Still another method is described in Canadian Pat. No. 870,369 issued May 11, 1971 to K. Gardner. With this method, the body is actually inverted during recovery so that it is held upside down in the saddle with the nose facing aft in the inboard stowed position. But like Canadian Pat. Nos. 879,530 and 1,005,702, the boom motion in Canadian Pat. No. 870,369 is also in the form of a long overhead arc in a vertical plane more or less parallel with the centreline of the ship.
A difficulty arises when attempts are made to fit the foregoing equipment into the restricted stern spaces of the smaller naval frigates and patrol vessels, and this difficulty is aggravated by the tendency in recent designs of such vessels to place a helicopter landing deck or pad above the stern spaces. For structural and other reasons, it is usually not permissible to pierce this landing deck or pad to gain working clearances. This means, that if one uses a means as described in the above-identified Canadian Pat. Nos. 870,369, 879,530 and 1,005,702, then large sections of the stern near the waterline must be cut into to gain working clearances below the helicopter deck or pad. Since cutting large sections of the stern out is equally unacceptable, a compromise in sonar performance usually has to be made in smaller vessels in that only smaller, lower range transducer arrays, bodies and handling equipment can be accommodated.
Thus, whereas there is room for shock absorbing over a limited vertical arc off the stern, the presence of the helicopter deck above prevents the boom being rotated forward past the vertical position to and from an inboard stow position.
A solution to this problem may be found in Canadian Pat. No. 1,120,790 issued Mar. 30, 1982 to Robert S. Norminton and assigned to Fleet Industries, a Division of Ronyx Corporation Limited. This invention sets forth an improvement in a system for launching, towing and recovering a towed body from a surface vessel, such system including a hoist sub-assembly and a boom sub-assembly to which boom bobbing means are provided for resiliently applying a torque to the boom member about the pivotal connection thereof to the vehicle commensurate with, and in response to changes in, a load applied to and/or the moving moment of, the assembled boom members, the improvement comprising: a boom subassembly comprising an inner boom and an outer boom, the outer boom being telescopable with respect to the inner boom, and boom telescoping means for resiliently extending and retracting the boom in response to changes in a load applied to the towing cable.
By one embodiment of that invention, the boom sub-assembly includes an inner boom and an outer boom. The outer boom may include wheels or rollers bearing on, and held captive by, the main members of the inner boom. On the other hand, rollers may be attached to the inner boom and be held captive by the main members of the outer boom. The outer boom is connected to the inner boom by one or more hydraulic cylinders. By introducing fluid into either the head or rod ends of these cylinders, the outer boom is made to telescope outward from the inner boom for normal towing, or to collapse into the inner boom, thereby creating a compact stowed combination of boom and towed body. The inner boom is connected to the turntable through a rotary actuator, and the combination of inner boom and outer boom is rotated in the vertical plane (i.e., either raised or lowered) by introducing fluid under pressure to this actuator.
The operation is as follows: The boom is lowered to put the saddle in the water to recover the body. The boom is then raised and collapsed (telescoped inward) for stowing the body. The boom (and indeed the entire hoist) is rotated 90.degree. to bring the body athwartships to the stowed position. The boom is then lowered over a slight arc to deposit the body into a support attached to the ship. The launch process is the reverse of that described above.
The telescopic boom is an essential part of the launch and recovery system. Its use is essential in order that everything is collapsed into a stowed length which is less than the beam of the stern transom.
While the above-described invention may solve the problem of lack of space, it suffers from the drawback that the turntable-mounted winch is, of necessity, mounted toward the corner of the stern, well away from the centreline of the ship. This results in off-centre towing, and at high speeds this may create problems in steering the ship.
With respect to the problems of shock absorbing, the main hazard involved in towing any device of substantial mass from a ship at sea is that of varying cable tensions due to in or out of phase undulations and speeds between the towing vessel and the towed mass. In the extreme, though not uncommon case, the towing cable is prone to falling slack then being followed by a snapping-to-taut condition. The transient cable load, at the instant the cable becomes taut, is high by several magnitudes when compared with the nominal towing load. Cable failures may result from such a situation.
When two interconnected masses are horizontally separated, one from the other, e.g., a tug pulling a ship, spring can be put into the tow line by the expedient of paying out a great deal of tow line, which for example, may be a cable. The weight of the tow line curves the span into a horizontal catenary curve and this together with an acceptable strain within the cable provides such a spring. In towing submerged massive bodies, for example, a sonar towed body, the problem of absorbing shock loads becomes much more difficult. With a submerged sonar body, the tow line has a tendency to be relatively straight and thus variable loads cause a corresponding variable strain in the tow line. In towing a submerged sonar body, from a hydrodynamic viewpoint, it is desirable to have low drag forces on the towing cable, and this may at least be partially achieved by use of fairing elements on the cable. The result of low drag characteristics and towing a submerged sonar body is that the towing cable extends more directly vertical along a straight line than otherwise. In a towing system of the latter type, it is difficult and often impossible to obtain sufficient internal spring within the cable itself to damp out transient loads. This is particularly so at slow ship speeds (in which case the cable more clearly approaches a vertical attitude) i.e., the slower the towing speed the lesser the internal spring and thus the more critical the problem. Transient loads in such a system follow the undulations of the floating vessel. On rough water, the intensity of the transient loads can be severe and particularly because of a snapping action occurring in the tow line resulting from the two bodies moving relative to one another at different speeds.
These effects are countered by providing a means to compensate for the motion of the vessel by allowing some of the tow cable to pay out while the stern of the vessel is rising and by causing some of it to reel in while the stern is falling. The methods by which this is done may be active, passive, or a combination of the two, and will be referred to here as "shock absorbing systems". A passive system is a simpler system and uses stored energy in the form of gas and mechanical springs. A passive system described in the aforementioned Canadian Pat. No. 879,530 utilizes gas-oil springs acting on the overboarding cylinders of the boom to obtain a "boom bobbing" type of shock absorption. This patent is described more fully later in the text.
The foregoing system is usually termed a "boom bobbing" system. It has the advantage that there is little or no cable excursion over the towing sheave during shock absorbing; and the disadvantages that in the presence of an overhead helicopter landing deck that there is insufficient room for boom travel, both for significant cable tension attenuation during shock absorbing, and for launch and recovery; and that of necessity the placement of the cylinders in such that response is non-linear and the mechanical advantage is poor.
Another suggestion is provided in U.S. Pat. No. 3,604,387 issued Sept. 4, 1971 to N. E. Hale, which provides a cable tensioning device which consists of another sheave carried on a pivotable arm which is moved towards and away from the winch by a piston and cylinder connected to an accumulator which maintains more or less constant pressure on the arm and thereby moves it in response to the increase and decrease in the tension of the cable when the acoustic body is towed. This suggestion suffers from the deficiency that it requires careful selection of various attachment points in the system to maintain the torque about the fulcrum constant, and also that it is prone to induce premature cable fatigue because of the back-and-forth cable excursions through the sheave.
The problem of providing shock absorbing in confined spaces was addressed in the aforementioned Canadian Pat. No. 1,120,790. This too describes a passive system.
By one embodiment of that invention, the boom sub-assembly includes an inner boom and an outer boom. The outer boom may include wheels or rollers bearing on, and held captive by, the main members of the inner boom. On the other hand, rollers may be attached to the inner boom and be held captive by the main members of the outer boom. The outer boom is connected to the inner boom by one or more hydraulic cylinders. By introducing fluid into either the head or rod ends of these cylinders, the outer boom is made to telescope outward from the inner boom for normal towing, or to collapse into the inner boom, thereby creating a compact stowed combination of boom and towed body. The inner boom is connected to the turntable through a rotary actuator, and the combination of inner boom and outer boom is rotated in the vertical plane (i.e., either raised or lowered) by introducing a fluid under pressure to this actuator.
In another embodiment of that invention, two shock absorbing/motion attenuating systems are used, each being capable of being used on its own, or being used together. Both systems may be connected to a common gas-oil spring system, but it is preferred that a separate such gas-oil system be used for each. One system is connected to the above-described rotary actuator connecting inner boom and turntable. The other system is connected to the cylinders connecting inner and outer booms. The primary system proposed here uses gas-oil springs connected to the rotary actuator of the inner boom to produce "boom bobbing" similar to, but on a smaller scale than, that described in the form in the above-identified Canadian Pat. No. 1,010,308. The secondary shock absorbing/motion attenuating system proposed here also utilizes gas-oil springs connected to the hydraulic cylinders connecting the inner and outer booms. Cable excursions into and out of the water are caused by collapsing the outer boom into the inner boom and by telescoping the outer boom from out of the inner under the action of hydraulic fluid transfer arising from the response of the gas-oil spring to variations in tow cable tension. This auxiliary system shock absorbing/motion attenuating is intended to supplement the main boom bobbing system at low speeds of the towing vessel, under which conditions telescoping the inner and outer booms will not cause undesirable surge of the towed body.
The main disadvantage with the combined shock absorbing system as described in Canadian Pat. No. 1,120,790 is that the auxiliary system utilizing the telescopic boom (not the boom bobbing system) suffers the same drawbacks as U.S. Pat. No. 3,604,387 in that it is prone to induce premature cable fatigue because of the back-and-forth excursion of the cable through the towing sheave.
With respect to the problems of tow-off, it is always desirable, when towing an underwater body in a straight line, that the body, cable and ship lie in the same vertical plane. This does not usually occur in practice. While fairings are often used on the two cable, instabilities, wake disturbances, cross currents or other effects may cause the cable to tow off to one side. When the ship is turning, the cable always tows off. Tow-off is usually measured as the angle the cable projected onto a vertical plane makes with a vertical line when viewed looking aft from the deck of the ship. When the ship is steaming in a straight line, mild two-off, that is, an angle of 10 or 15 degrees of the vertical as defined above, is usually acceptable. Tow-off above 15 degrees is somewhat objectionable because body depth is sacrificed, and the body itself may be out of position. This is important because while the sonar array within the body will signal the position of the target with respect to the body, if the position of the body with respect to the ship is not known, then neither is the position of the target with respect to the ship. Severe tow-off of 30 degrees or more is most objectionable, particularly if the ship is rolling in heavy seas, as this may cause the cable to bear heavily against the towing structure (saddle or boom), resulting in wracking of the structure and possible severe damage to the cable. Various means have been adopted to protect the cable, e.g., guide plates, as presently used on the AN/SQA 502 variable depth sonar currently in use in the Maritime Command of the Canadian Armed Forces; cable guide assemblies, as taught in Canadian Pat. No. 1,005,702 issued Feb. 22, 1977 to A. Kemeny, assigned to Ronyx Corporation Limited; and floating roller boxes, as taught in Canadian Pat. No. 923,378 issued Mar. 27, 1973 to N. E. Hale, granted to Fathom Oceanology Limited. All of these devices protect the fairings and cable to a greater or lesser degree, but do nothing to relieve wracking of the towing structure. Canadian Pat. No. 1,093,061, issued Jan. 6, 1981 to R. S. Norminton, assigned to Ronyx Corporation Limited, describes a fairlead sheave and saddle assembly which completely protects the cable and fairings, and relieves much of the towing structure from the wracking associated with tow-off forces. Details of this patent will be discussed more fully hereinafter.
With respect to the problems of fleet angle and fairing training, it is recognized that, in winch applications, it becomes necessary to reduce the fleet angle in order to obtain a proper laying of adjacent turns of cable upon the winch drum. For accurate reeling of cable, the cable should approach the drum at right angles which is to say, the fleet angle should be as close to zero as practicable.
Numerous methods of achieving a fleet angle as close to zero as practicable exist in mechanisms known as level winders. These in general comprise a travelling guide which moves laterally relative to the cable, i.e., in a direction parallel to the winch drum pivot axis, one pitch (distance between turns) for every revolution of the winch drum and through which the cable must pass prior to reaching the winch. Typical examples of movable guides for reducing the fleet angle are found in the structures illustrated in Canadian Pat. Nos. 692,070 and 707,634 issued, respectively, Aug. 4, 1964 and Apr. 13, 1965.
In towed sonar hoist systems, faired cables are generally employed and a fully reeled cable generally comprises a single layer upon the winch drum. In the case of towed sonar cables, the problem of spooling becomes more tenuous on account of the fairing that it carries for hydrodynamic anti-drag purposes.
Prior attempts have also been made to eliminate the cable guides in which case it has been proposed to move the winch drum during winding in the cable as exemplified by Canadian Pat. No. 655,052 issued Jan. 1, 1963 to Spider Staging, Incorporated. In the patented device, movement of the winch drum is responsive to tension applied to the cable, and, accordingly, the fleet angle in such structure will vary proportionally to changes in load. Such an arrangement is satisfactory where the cable is subjected to substantially contant loads. It is, however, unsatisfactory where the tension varies considerably in the cable, say, from winding in at one time to the tension in the same cable when being wound in at another time, and also where there is considerable variation in the tension in the cable during a winding operation.
Another successful means of keeping the fleet angle as close as possible to zero is taught in Canadian Pat. No. 1,005,702. By means of a careful orientation of the winch drum relative to the boom sheave throughout its travel and by means of limiting the width of the winch drum and by means of offsetting the boom assembly between 2.degree. and 3.degree., preferably by 2.degree.30', the necessity of using any separate spooling apparatus is avoided. Consequently, from the extremes of having the boom fully extended with all cable either in or out, to bringing the boom fully inboard with all cable wound on the drum, the fleet angle is less than the critical 3.degree. figure. This means that separate cable spooling is unnecessary and that the cable will wind onto the winch drum without further complication.
In many instances, experience has taught that the critical fleet angle is less than 3.degree., as low as 11/2.degree. or even less. This may be achieved through the use of level winders, which are troublesome and space consuming, or by one varient of Canadian Pat. No. 1,111,829 issued Nov. 3, 1981 to R. S. Norminton, assigned to Ronyx Corporation Limited, wherein the winch base is mounted on rollers and the entire winch translates linearly to minimize fleet angle.
Closely associated with the problem of fleet angle is the problem of fairing training. For while it is necessary to have a low fleet angle to ensure the faired cable winds onto and into the drum grooves properly without jumping, it is also necessary that the freely swivelling fairings remain upright as they are wound on in order that fairings in adjacent turns clash and damage or destroy one another. The problem of providing a fairing training device in conventional towing systems is made even more diffeicult by the fact that the cable never contacts the drum at the same angular positions. A look at FIG. D, included here from aforementioned Canadian Pat. No. 879,530 will confirm this. A line struck from the top of the towing sheave to the drum for every boom position shown will contact the drum at a different tangent point. This means that any fairing training device, in addition to fleeting across the face of the drum, must also follow around the drum.
With respect to the problems of storage of long tow cables, in towed sonar hoist systems, faired cables are generally employed and a fully reeled cable generally comprises a single layer upon the winch drum. In the case of towed sonar cables, the problem of spooling becomes more tenuous on account of the fairing that it carries for hydrodynamic anti-drag purposes.
The tailpieces of the segmented fairings are almost always made of light-weight plastic, and it is not possible to spool the cable in multiple layers onto a drum without crushing the fairings in all but the top layer. This means that, in usual practice, only single-layer winding could be used with a segmented fairing cable. If the cable is very long, the winding drum may be huge. This would cause topside weight and space problems, fleet angle problems and might necessitate the use of extra power.
A number of methods of circumventing these problems have een proposed in the past. For example, U.S. Pat. Nos. 2,397,957 issued Apr. 9, 1946 to H. B. Freeman; 2,401,783 issued June 11, 1946 to K. W. Wilcoxon; 3,209,718 issued Oct. 5, 1965 to R. L. Rather et al; and 3,241,513 issued Mar. 22, 1966 to R. L. Rather et al, all attempt to solve the problem by the use of removable fairings. With such fairings, the base cable can be spooled as a multi-layer onto a storage drum. However, a major disadvantage is that time is consumed stripping the fairings on cable recovery and installing the fairings during cable payout. This could be particularly difficult in high sea states. A problem also arises in storing the removed fairings without damage.
Canadian Pat. No. 902,577 issued June 13, 1972 to N. E. Hale proposed to solve the problem by using multiple concentric drums. However, there are many disadvantages inherent in a drum of this construction. Firstly, the outer drums must be slotted across the face of the shell and this may severly weaken the drums. Secondly, the cable or fairings or both may be severely damaged at the points of inflection in bridging the shell gaps. Thirdly, in one embodiment, one drum is connected to the other by short-stroke hydraulic cylinders connected up to a manifold system with quick-release connections. Frequent use of this method aboard ship will result in hydraulic spills, and contamination being introduced into the hydraulic system. In another embodiment, the outer drum is given motive power by wedging up the tailpieces of the fairings into contact with the roof of an access chamber in the outer drum. This may damage and crush the fairings.
It has also been suggested to use two concentric drums with unbroken shell faces which screw into one another. The major disadvantage of such proposal was that with all the cable paid out, the drums must be completely unscrewed, and in this condition they take up as much space and weight as the one single layer drum previously referred to and with a great increase in complexity.
Another proposal is shown in Canadian Pat. No. 671,172 issued Sept. 24, 1963 to Nautec Corporation which provided a level winding device disposed at right angles to a cable storage drum. A key feature of this invention was the use of pressure rollers to exert a squeezing force on the cable. It is virtually impossible to exert such a force on cable enclosed with segmental fairings for the purpose of gaining traction. In addition, such squeezing force might damage the fairings, which are somewhat fragile.
Yet another proposal was shown in Canadian Pat. No. 949,547 issued June 18, 1974 to American Chain and Cable Co. Inc. which provided a cable trained over a double capstan, with its other end extending through a guide into a cylindrical container disposed at right angles to the capstan. Key features of this proposal were the use of separate traction and storage drums. The storage drum and its drive alone would take up as much space as a single simple powered drum used for both power and storage. In other words, and aside from other drawbacks, as a means of spooling extra long lengths of segmentally faired cable (all in one single layer), the traction winch with separate storage drum is the most space-consuming solution of all, and there would be no room for it aboard most naval vessels.
A solution to the problem of storing very long lengths of faired tow cable without putting the cable itself at risk was taught in U.S. Pat. No. 4,312,496 issued Jan. 26, 1982 to R. S. Norminton, assigned to Fleet Industries. By such invention, a compound drum hoist was provided comprising: (a) a first cable-spooling drum rotatably and drivingly mounted on a first shaft; (b) a second cable-spooling drum nested within the first drum, and rotatably mounted on a second shaft disposed at an angle of 90.degree..+-.30.degree. but preferably at right angles to the first shaft; (c) means for rotating the second drum while keeping the first drum stationary, thereby to spool faired cable onto the second drum; and (d) means for substantially simultaneously rotating the first drum along with the second drum, thereby to spool faired cable onto the first drum.
Experience has shown that use of such method will reduce space somewhat, but at the expense of increased winch complexity.
With respect to the problems of storage of spare cable, it has been found, in practice, that the tow cable progressively deteriorates in service due to the combined effects of fatigue and corrosion. It has also been found that this deterioration is worst at the attachment point of the towed body to the tow cable. It is common practice when this happens to shorten the cable by cutting off a length of deteriorated end, reterminating it and re-attaching it to the towed body. However, only a few shortenings and reterminations can be made before the cable becomes too short to be serviceable. Accommodating extra cable on the deck handling equipment to allow a greater number of shortenings is space consuming, further aggravating the problem of shortage of space.
By the invention provided by Canadian Pat. No. 1,111,829 issued Nov. 3, 1981 to Robert S. Norminton, and assigned to Fleet Industries, a Division of Ronyx Corporation Limited, an improved hoist drum assembly was provided which included means for storing large excesses of faired towing cable for multiplicity of shortenings and reterminations, required to extend service life, which included a cable winder assembly associated therewith, and which included means for controlling the fleet angle of the cable as it is wound thereon. An improvement is provided in a system for launching, towing and recovering a towed body from a towing surface vessel using faired towing cable, the system including a hoist sub-assembly and a boom sub-assembly, the improvement comprising: a main winch drum in the hoist sub-assembly for storing live turns of the faired towing cable wound in a single layer in a first direction, which faired cable is adapted to be wound on and unwound off the main winch drum, the main winch drum including a slot magazine recessed into the interior thereof at one end therefor for storing dead, unfaired towing cable wound in multilayers in the same first direction thereof; the main winch drum including a single cable clamp completely inboard of all turns of the faired and unfaired cable and near the inboard end of the cable.
Thus, by an embodiment of that invention, the hoist sub-assembly includes a winch drum with a special annular and radial slot magazine at one end. This annular and radial magazine allows for spooling of a large excess of unfaired cable in several layers and takes up no more room than would be required by the normal number of "dead" turns left on a conventional drum for safety when all the faired cable has been paid out. The dead turns are wound on the slot magazine before the live faired turns have been wound on the winch drum. This annular and radial slot magazine allows for a multiplicity of shortenings and reterminations at the towed body and a significant prolongation of cable service life over that currently obtained. As the cable is shortened dead turns may be transferred to active turns witout release of either cable load or cable clamp during which filler pieces are introduced into the slot beneath the reduced number of layers.
With respect to the problems of cable tension measurement, it has long been wished that a continuous monitoring of cable tension could be obtained. This is particularly true in high sea states. to detect the onset of dangerous towing conditions. All AN/SQS-505 towed bodies currently in use in Maritime Command of the Canadian Armed Forces are now equipped with strain-gauged tow points, but the tension so measured is the lowest in the tow cable. Tension increases progressively up the cable, and is highest at the towing sheave at the ship. Since cable tensions at body and ship may change constantly one with respect to the other, ship-end tension cannot be considered to be some constant multiple of body-end tension. Ship-end tension cannot be measured directly at the cable, because no method has as yet been devised to grip the electromechanical cable without damaging or destroying the crush-sensitive electrical core. It cannot be measured accurately at the winch by means of pressure gauges or transducers, because they are too many frictional losses between winch or winch motor and towing sheave. In the system disclosed by Canadian Pat. No. 879,530, tension can be measured with great difficulty and complexity by strain-gauging the towing sheave shaft, boom or deflection roller. The complexity arises because strains so measured represent the vector sum of tensions around a sheave. As the angle of cable wrap changes, so does the vector sum even though the cable tension itself does not. The angle of cable wrap and the tension are forever changing with a boom-bobbing type of shock-absorbing system. Thus, any method of measuring cable tension on a system similar to that in Canadian Pat. No. 879,530 must measure vector sum of tensions around a sheave, angle of wrap, and possibly boom angle as well, electronically combining and processing all these output signals to arrive at a final true cable tension.
Cable tension at the ship becomes of primary importance when it is used as a primary reference signal for active or partially active shock-absorbing means. It is possible to use acceleration methods instead, but accelerometer signals must be further processed to be useful. Since the purpose of shock absorbing is to attenuate cable tension variations, it is far better to use cable tension directly as a primary reference signal for a closed loop feedback servo type of active or partially active shock-absorbing system.